Strain Rate Dependent Ductility and Strain Hardening in Q&P Steels

Abstract

Due to their high strength, formability and affordable cost, quenched and partitioned (Q&P) steels have shown the potential to reduce the mass of vehicles, thereby decreasing fuel consumption during service. Furthermore, because a lower mass of steel is used in each vehicle, energy consumption associated with the steelmaking process is also reduced. Q&P steels utilize the deformation-induced martensitic transformation (DIMT) of metastable retained austenite to enhance ductility and strain hardening. Accordingly, improvement of mechanical performance is contingent on the ability to precisely control the chemical and mechanical stability of austenite. Considering the multitude of factors that influence austenite stability, optimizing microstructures to delay necking or fracture is challenging, particularly as temperature and strain rate increase. Tensile tests of an intercritically annealed C-Mn-Si Q&P steel were performed over a range of strain rates (10 À4 to 10 À1 s À1 ) to evaluate effects on the DIMT and sheet tensile properties. As strain rates increased from 10 À4 to 10 À1 s À1 , the uniform elongation decreased from approximately 19 to 14 pct. This reduction in uniform elongation is associated with a decrease in the strain hardening exponent near the onset of strain localization. Based on experimental data from this study and review of previous research, it is postulated that the strengthening contribution of DIMT is controlled by competing effects of: (i) a decreasing chemical driving force for DIMT caused by deformation-induced heat accumulation at higher strain rates and (ii) an increasing number of martensite nucleation sites. This suggests that tailoring austenite stability for specific deformation conditions could enable further optimization of formability and vehicle crash behavior.

Decoupling the Impacts of Strain Rate and Temperature on TRIP in a Q&P Steel

“…For the 10 À1 s À1 (adiabatic) specimen, the surface temperature increased as a function of e, up to approximately 40°C above room temperature at the end of uniform elongation. Based on previous work by the authors, 9 it is assumed that the strain rate of 10 À1 s À1 is ostensibly greater than the adiabatic threshold strain rate for QP980. The thermal profile of the 10 À4 s À1 (resistance heated) samples was chosen empirically, based on the temperature of the 10 À1 s À1 (adiabatic) specimen.…”

“…Serrations in the strain hardening exponent curves were observed for specimens deformed at 10 À4 s À1 for both thermal conditions; these are attributed to dynamic strain aging in ferrite and secondary martensite. 9,15,16 It is known that the diffusivity of carbon within the BCC matrix increases with increasing temperature. However, between the isothermal and resistance heated specimens, no difference in the severity of the serrations was observed.…”

“…QP980 was revealed to exhibit strain rate-sensitive ductility and strain hardening; as the strain rate was increased to 10 À1 s À1 , the uniform elongation decreased to 14%. 9 To investigate the strain ratesensitive ductility, this study uses the same QP980 steel.…”

“…We showed previously that increasing the strain rate from 10 À4 s À1 to 10 À1 s À1 deleteriously decreased the strain hardening rate and ductility of a Q&P steel. 9 It was hypothesized that a ductility trough existed at intermediate strain rates (approximately 10 À1 s À1 ), which was caused by the competing effects of adiabatic heating and strain rate on the deformation-induced martensitic transformation. That experiment was limited in two ways.…”

Interpretation of Dynamic Strain Aging in an Intercritical Annealed Steel by Dislocation Multiplication Induced by Martensitic Transformation

“…[51] However, an adiabatic temperature increase will decrease the chemical driving force for DIMT, contributing to the inhibition of coalescence and propagation of martensitic embryos. [51][52][53] There are two competing factors for transformation amount of RA. The characterization of the transformation kinetics of RA identified by the interrupted tensile tests in Figure 4, confirmed that the inhibition of martensitic embryos growth caused by adiabatic temperature increase plays a predominant role.…”